Electrical power drain and highly sensitive reception of radio signals have been critical design parameters of portable electronic devices such as pagers and personal communication devices for many years.
When an electronic device is used to receive a radio signal in a communication system in which the radiation (transmission) of electromagnetic energy is reflected and absorbed by numerous objects, such as buildings, vehicles, and trees, the resulting electromagnetic energy environment is described as a fading environment, and the components of the electromagnetic energy waves that are orthogonally polarized have low correlation of their amplitude and phase. A known technique of increasing the sensitivity of electronic devices to the reception of such radio signals in such systems is to use two receiving antennas arranged so as to respond to substantially orthogonal polarizations of the electromagnetic energy, such as linear polarizations 90 degrees apart, carrying the radio signal. When two antennas are used that intercept polarizations of the electromagnetic waves that are approximately 90 degrees apart, the radio signal intercepted by a second one of the two antennas can be used to augment the radio signal intercepted by a first one of the two antennas. This is called diversity reception.
Although the radio signals intercepted by the two receiving antennas can be added together to increase the average strength of the intercepted energy coupled to a receiver, doing so in a manner that avoids situations wherein the two signals partially or wholly cancel each other when their phases are approximately or exactly 180 degrees apart is technically complex. That technique is know as diversity combining. A very common alternative approach for using two such antennas, which is simple and works well, is to select the antenna that has a stronger signal. When this technique is used, a means to switch the antennas is needed, as well as a method to select the stronger signal.
A common technique for switching the antennas is to use PIN diodes. In this technique, one PIN diode is used to turn one antenna on while a second PIN diode is used to turn the other antenna off, until the switching is reversed. Because only one signal is received at a time, an algorithm is used to cause switching when the presently received signal is too weak, or upon another occasion (such as a timed selection). The PIN diode that is off is reversed biased and therefore draws only leakage current. The PIN diode that is on, though, is forward biased and typically draws on the order of 1 to 10 milliamps. Such a technique has been used for over ten years, but this much current drain is substantial in a product such as a pager.
Therefore, what is needed is a means for achieving a switched diversity antenna that draws substantially less than 1 milliamp.